1. Field of the Invention
This invention relates to a high-frequency heating apparatus having an inverter circuit, and more particularly to a high-frequency heating apparatus whose output heat can be set over a broad range and which is suitable for use as a cooking apparatus such as a microwave oven.
2. Description of the Related Art
As is known in the art, a type of cooking apparatuses such as microwave ovens, which has a high-frequency heating device, has an inverter circuit for supplying drive power to the heating device. Such a cooking apparatus is disclosed in U.S. Pat. No. 4,724,291. This cooking apparatus also has, besides an inverter circuit, an oscillator circuit for generating a sawtooth-wave signal, a pulse-width modulating circuit for performing pulse-width modulation on the sawtooth-wave signal in accordance with a signal setting the output of the heating device, and a drive circuit for turning on and off the switching element of the inverter circuit in accordance with the output of the pulse-width modulating circuit. Therefore, the output of the heating device can be continuously controlled over a broad range. Due to the use of the inverter circuit, it is sufficient for the cooking apparatus to have a small, light high-voltage transformer. Hence, the cooking apparatus can be compact as a whole.
However, the cooking apparatus of the type described above has a drawback. Since the oscillator circuit and the pulse-width modulating circuit process analog data to control the inverter circuit, the output of the heating device will be different from the desired value if the constants of the oscillator circuit and the pulse-width modulating circuit differ from the design values. When the heating device outputs more or less heat than desired, the cooking apparatus cannot cook food properly.
When the constants of the oscillator circuit and the pulse-width modulating circuit differ very much from the design values, a voltage higher than the rated one, or a current greater than the rated one is applied to the switching element of the inverter circuit, inevitably breaking down the switching element. To prevent the breakdown of the switching element, a circuit can be used which changes the constants of the oscillator circuit and the pulse-width modulating circuits to the design values. The use of this additional component renders the cooking apparatus complex in structure, and raises the manufacturing cost of the apparatus.
The above problem is also inherent in a so-called "electromagnetic induction cooking apparatus" which has an analog-controlled inverter and which performs high-frequency induction heating.